The present invention relates to nuclear magnetic resonance (NMR) sample investigation and, more particularly, to novel methods for the inversion of the magnetization by non-linear adiabatic fast passage (AFP) to minimize the radio-frequency (RF) power dissipated in the sample to be investigated.
Many NMR techniques, such as inversion recovery, spin echo and the like which may be utilized to provide a human-viewable image of a sample, require the inversion of the magnetization M vector. NMR magnetization inversion is usually achieved by application of a 180.degree., or a .pi.-radian, RF pulse at the NMR resonance, or Larmor, frequency. However, use of a 180.degree. inverting pulse requires a high degree of homogeneity in both the static magnetic field and the RF magnetic field if relatively low distortion of the final image is to be generated. It is well known that the magnetization vector inversion technique called adiabatic fast passage (AFP) can generally produce magnetization inversion which is much less sensitive to inhomogeneity in either or both of the main static magnetic field B.sub.O or the RF magnetic field B.sub.1. The AFP inversion technique applies a strong RF pulse while sweeping either the static magnetic field magnitude B.sub.O or the RF field instantaneous frequency .omega. linearly through resonance while meeting the condition, first described by Bloch in 1946, that: EQU B.sub.1 /T.sub.2 &lt;&lt;(dB.sub.O /dt)&lt;&lt;.epsilon.B.sub.1.sup.2 ( 1)
where T.sub.2 is the spin-spin relaxation time and .gamma. is the gyromagnetic ratio of the nuclear specie to be imaged. The use of AFP provides increased accuracy margins in many NMR proceduresl; however, the AFP technique requires deposition in the sample of more than 10 times as much RF power, relative to the amount of power required for magnitization inversion by a 180.degree. RF pulse, and is thus not seriously considered for NMR experiments upon relatively sensitive living tissue samples. Since it is relatively difficult to sweep the static magnetic field magnitude B.sub.O, especially in an NMR medical imaging system, it is desirable to provide an RF frequency sweep for adiabatic-fast-passage magnetization vector inversion, which not only has a relative insensitivity to any inhomogeneitics in the static magnetic B.sub.O or RF magnetic B.sub.1 fields, but also minimizes the required RF power magnitude.